1. Field of the Invention
The present invention relates generally to an improved process for the separation of insoluble solid materials from slurry resulting from the dissolution of calcined trona ore in water or unsaturated aqueous sodium carbonate solution in the production of soda ash.
2. Description of the Prior Art
Trona is a naturally-occurring material found in abundance in Sweetwater County, Wyoming. It is extracted from seams lying 800 to 1800 feet underground using well-established mining techniques. Trona consists of sodium sesquicarbonate (NaHCO.sub.3.Na.sub.2 CO.sub.3.2H.sub.2 O), insoluble sand, shale, etc., and free moisture. The approximate percentages of these constituents in trona is as follows:
______________________________________ Sodium sesquicarbonate 85-92% Insolubles 6-13% Water 0.5-3.0% ______________________________________
Trona is used in the manufacture of soda ash (Na.sub.2 CO.sub.3). It is essential that manufactured soda ash be free of insoluble materials because the presence of such materials is highly objectionable to soda ash users.
In the production of soda ash from natural trona ore, two methods are most prevalent in the industry. In one of these methods, known as the "monohydrate" process, trona ore is first heated to expel water and carbon dioxide to produce an impure soda ash. The sodium carbonate values of this soda ash are then dissolved in an aqueous medium from which insoluble impurities are removed. Sodium carbonate monohydrate is then crystallized from the purified solution and these crystals are then heated to remove water and produce soda ash.
More specifically, the first step in manufacturing soda ash by the monohydrate process after the trona ore has been reduced to the proper size, i.e., minus one-quarter (-1/4) inch, is to heat the trona ore in a calciner so that the following reaction takes place: EQU NaHCO.sub.3.Na.sub.2 CO.sub.3.2H.sub.2 O.fwdarw.3/2Na.sub.2 CO.sub.3 +1/2CO.sub.2 +5/2H.sub.2 O
The heated ore produced by this reaction consists of soda ash, Na.sub.2 CO.sub.3, and insoluble material. This ore is then mixed with water or unsaturated sodium carbonate solution to form a slurry comprising aqueous sodium carbonate in the liquid phase and insoluble gangue and some undissolved soda ash in the solid phase. The foregoing process steps are well-known and established practice in the industry.
The objective of the next series of operations in the manufacture of soda ash is to produce a solution which is completely free of insoluble materials. This solids-free liquid is then fed to crystallizers in which sodium carbonate monohydrate crystals (Na.sub.2 CO.sub.3.H.sub.2 O) are produced. These crystals are then deliquored in a centrifuge, are dried to remove free moisture and water of hydration in a dryer, are sized by well-known methods, for example, by means of a vibrating screen and air classifier, and are thus converted into saleable anhydrous sodium carbonate. Users of this latter material require that the soda ash be at least 99.5% Na.sub.2 CO.sub.3 and contain, for all practical purposes, no insoluble material. Thus, solids removal from the precursor solution prior to the crystallization step in the production process is essential.
The first step in the solids removal operation is to form from the initial slurry of insolubles and undissolved sodium carbonate, which is produced when the calcined trona ore is mixed with water or unsaturated sodium carbonate solution, two slurries one of which contains the bulk of the larger, coarser particles, i.e., those coarser than about 70 mesh, and the other of which contains the bulk of the finer particles. The accepted practice in the industry is to separate coarser insolubles, those having a size of approximately plus (+) 70 mesh, minus one-quarter (-1/4 inch, from the finer insoluble materials by means of a spiral or screw classifier with either water or a weak soda ash solution. The coarse insolubles are separated from the resultant solution, again by means of a screw or spiral classifier, and are then discarded. The weak solution is then returned to the process to be brought up to full saturation, i.e., approximately 30% Na.sub.2 CO.sub.3 and 70% H.sub.2 O, by mixing the solution with calcined trona ore. The finer solids and the saturated solution then proceed to subsequent settling and filtration operations which remove substantially all of the remaining solids.
Although the separation of the insoluble materials as described above is adequately achieved by using a screw or spiral classifier, such classifiers provide only limited flexibility over the management of the solids separation during the production of soda ash. Spiral classifiers and rake classifiers are single units which are used for a single separation operation in present methods of producing soda ash from trona. These classifiers do not provide the versatility of coping with fluctuations in feed rate, feed slurry solids content, or solids size distribution. The foregoing three parameter variations are experienced in day-to-day manufacturing of soda ash due to the fluctuations in soda ash demand, variation in insoluble content of the mined trona, and in size distribution of the trona ore.